1. Field of the Invention
The present invention relates to multiple wireless-radio technology communications systems, and, in particular, to a communications scheme wherein a wireless device is adapted to communicate with disparate wireless services.
2. Description of the Related Art
Reliable and efficient transmission of information signals over imperfect communication channels is essential for wireless communication systems. One method of transmission is multiple-radio transmission, which increases the spectral efficiency of a wireless communications system. Multiple-radio transmission includes SIMO (single input, multiple output), MISO (multiple input, single output), and MIMO (multiple input, multiple output). In SIMO technology, one antenna is used at the source, and two or more antennas are used at the destination. In MISO technology, two or more antennas are used at the source, and one antenna is used at the destination. In MIMO technology, multiple antennas are employed at both the source and the destination.
In a typical MIMO transmission scheme, a transmitter employs two or more radios (transceivers) to send separate signals on two or more corresponding transmit antennas. The separately transmitted signals are combined as they pass through the channel, and a multiple-radio receiver receives the combined signals on each of two or more receive antennas. The receiver detects, demodulates, and further processes the received signals to extract the information.
In both single-radio and multiple-radio transmission schemes, a wireless device typically communicates with a plurality of cells that are served by base stations in a terrestrial or satellite cellular system. A typical cellular system may include hundreds of cells and may serve thousands of wireless devices. The cells generally serve as nodes in the system from which links are established between wireless devices and a Mobile Telephone Switching Office (MTSO) by way of the base stations serving the cells. Each cell may have allocated to it (i) one or more dedicated control channels, which are dedicated channels used for transmitting cell identification and paging information, and (ii) one or more traffic channels, which carry voice and/or data information. Through the cellular network, a duplexed radio communication link may be effected between two wireless devices or, through a Public Switched-Telephone Network (PSTN), between a wireless device and a landline device.
Several types of access techniques are conventionally used to provide wireless services to users of cellular systems. Traditional analog cellular systems generally create communications channels using a system referred to as Frequency Division Multiple Access (FDMA), wherein discrete frequency bands serve as channels over which wireless devices communicate with base stations. Typically, these bands are reused in geographically-separated cells in order to increase system capacity.
Modern digital cellular systems typically utilize different multiple-access techniques, such as Time-Division Multiple Access (TDMA) and/or Code-Division Multiple Access (CDMA), to provide increased spectral efficiency. In TDMA systems, such as those conforming to the GSM or IS-136 standards, carriers are divided into sequential time slots that are assigned to multiple channels, such that a plurality of channels may be multiplexed on a single carrier. CDMA systems, such as those conforming to the IS-95 Standard, achieve increased channel capacity by using “spread-spectrum” techniques, wherein a channel is defined by modulating a data-modulated carrier signal by a unique spreading code that spreads an original data-modulated carrier over a wide portion of the frequency spectrum in which the communications system operates.
Conventional spread-spectrum CDMA communications systems commonly use “Direct Sequence” (DS) spread-spectrum modulation. In DS modulation, a data-modulated carrier is directly modulated by a spreading code or sequence before being amplified by a power amplifier and transmitted. However, other forms of spread-spectrum modulation may be used.
When a wireless device in a cellular system is turned on, it generally searches for possible base stations with which to synchronize. In cellular systems, there are a number of possible radio channels or frequencies that the base stations can use, and the wireless device may have to scan all of these frequencies in order to find the best base station to use, in terms of signal strength and/or capacity. For example, in Wideband CDMA (WCDMA), there are about 300 possible radio channels separated by about 200 kHz, at about 1.9 GHz (uplink) and about 2.1 GHz (downlink).
In WCDMA, a wireless device uses certain control channels transmitted from the base stations to find and detect a cell. These control channels are known as the Primary Synchronization CHannel (P-SCH), Secondary Synchronization CHannel (S-SCH), and Common Pilot CHannel (CPICH). In general, the initial cell search procedure may work as follows: (i) P-SCH is used in order to detect a new cell; (ii) if a new cell is detected, S-SCH is used to find the timing and scrambling code for the new cell; and (iii) when the timing for the new cell is found, CPICH is used to measure the signal strength.
In principle, the wireless device may need to perform the cell search on each radio channel in order to be certain all base stations have been found. From these base stations, the best base station to use may be selected for use.
When performing the initial cell search in systems such as WCDMA, it may take a long time to synchronize with the base station. In particular, because of a large number of radio channels, it may take a long time to perform a frequency scan and a cell search. The speed of detecting multiple carriers and frequency bands is important in quickly finding a cell. Because of the length of time to perform such operations, a wireless device can often take between 30 seconds and several minutes to be in service after powering on or returning to a coverage area. This time period can be further increased when a wireless device that supports several bands and multiple-radio access technologies sequentially searches cells.